History[04]

The Kylin project was started in 2013, from eBay's R&D in Shanghai, China. It was open sourced on Github as "KylinOLAP" in Oct 2014.

In Nov 2015, Kylin joined Apache Software Foundation incubator;

In Dec 2015, Apache Kylin became a Top Level Project.

Compression[07]

Kylin applies dictionary encoding to all dimension values in data cubes. Kylin's dictionary is order-preserving and supports mapping both from keys to values and vice versa. The dictionary is implemented as a radix tree. Each node in the radix tree also contains the size of its subtree to support mapping values back to keys.

Besides, Kylin also supports naive compression algorithms in HBase and Hive.

Data Model[08]

Data cubes are essentially HBase tables. Given a dimension column set, Kylin pre-aggregates all possible combinations of their attributes by map-reduce jobs, then encode the dimensions with dictionary encoding. Finally, Kylin encodes all data cubes to Rowkeys in HBase. The format of a Rowkey is cuboid id + attribute. For example, assume a data cube on year and city with cuboid id 00000001, and there is a row year=1994, city=Beijing, sum(sales)=100, and a dictionary maps 1994=0, Beijing=1, there will be an entry in the HBase table Rowkey=00000001+01, value=100.

Foreign Keys[09]

Kylin supports star schema and snowflake schema. A user needs to specify fact tables and lookup tables before building cubes. Kylin pre-joins the tables when building data cubes.

Joins[10]

On cube building phase, Kylin use Hive to pre-join the fact table and lookup tables.

On query time, table joins are supported by the Apache Calcite query engine. Calcite will decompose the join operator several single table lookup operators and each of them will be completed by Kylin.

Query Compilation[11]

The Apache Calcite query engine does code generation for SQL queries.

Query Execution

The Apache Calcite query engine is used to parse, generate and optimize execution plans.

Query Interface[05]

Kylin supports a subset of Apache Calcite's supported queries. Since Kylin is a pure OLAP engine, it only supports SELECT queries. INSERT, UPDATE and DELETE are not supported.

Storage Architecture[12]

Kylin stores the data cubes in HBase, and stores metadata in HBase or MySQL (MySQL metastore is still under test).

Storage Model

Kylin stores its data in key-value store.

System Architecture[13]

Kylin relies on Hive to store raw tables and HBase to store data cubes, both of which store data on HDFS.